Lithium-ion secondary batteries are widely used as power sources of portable electronic devices. In such a lithium-ion secondary battery, a composite oxide such as LiCoO2 and LiMn2O4 is mainly used as a positive electrode active material, and a graphite-based material is used as a negative electrode active material. Since both these materials have small irreversible capacity, the battery can provide battery capacity without a great decrease from positive electrode capacity or negative electrode capacity.
In recent years, a silicate material Li2MSiO4 (M=at least one selected from the group consisting of Fe, Mn, and Co) has attracted attention as a positive electrode active material in view of high safety and the amount of resources. However, unfortunately, the silicate material has a larger irreversible capacity than conventional composite oxides such as LiCoO2 and LiMn2O4. Therefore, when a battery comprises a positive electrode using a silicate material having a large irreversible capacity as a positive electrode active material and a negative electrode using a graphite-based material having a small irreversible capacity as a negative electrode active material, disadvantageously, actual capacity of the negative electrode decreases.
On the other hand, research and development have been conducted on tin-based materials and silicon-based materials having larger capacity than the graphite materials as negative electrode active materials. Unfortunately, the tin-based materials and the silicon-based materials have large irreversible capacity. Therefore, when a battery comprises a positive electrode using one of the abovementioned composite oxides having a small irreversible capacity as a positive electrode active material, and a negative electrode using one of the abovementioned tin-based materials and the silicon-based materials having a large irreversible capacity as a negative electrode active material, unfortunately, actual capacity of the positive electrode decreases. In order to improve this problem, it is possible to predope lithium on the one of the tin-based materials and the silicon-based materials and thereby reduce the irreversible capacity. However, this is disadvantageous in battery mass production due to an addition of the predoping step.
Moreover, Japanese Unexamined Patent Application Publication No. 2000-173,586 has proposed to make most of reversible capacities of a positive electrode and a negative electrode and enable battery capacity enhancement by adding, to the negative electrode, a metal compound to be electrochemically reduced to a metal upon electric charge. However, this is not suitable for battery mass production because of addition of a step of adding the metal compound to the negative electrode.
Japanese Unexamined Patent Application Publication No. 2011-228,052 is based on a finding that cations other than Li ions move from a positive electrode active material Li2MnO3 to a negative electrode by initial electric charge, and has proposed to reduce battery weight and size without decreasing battery capacity by setting actual capacity of the negative electrode to be smaller than actual capacity of a positive electrode by decreasing the amount of a negative electrode active material from a conventional amount.